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Troubleshooting the SB II

John AE5X
 

After completing the assembly I applied power to my SB II to begin the alignment. Unfortunately, I only heard a soft 'pop' in the headphones, then silence and nothing displayed on LCD.

My plan was to measure DC at all places it should be, working backwards from the LCD and U8 (ATMEGA328P). I didn't get far before I found that I have zero volts out of U7 despite having 5VDC into that IC on Pins 1 and 3. I measured resistance from its output (Pin 5) to ground, thinking that I may have shorted it to ground with solder but that is not the case. I reflowed the solder on the pin anyway and verified that it wasn't shorted to ground - it wasn't.

Still, good inputs on pins 1 and 3, no output on pin 5.

I highly doubt that such a component could go bad - am I overlooking something?

Tnx/73,

John AE5X

John AE5X
 

The area:

 

Potential shorts across I2C signals on pins 4/5 on the Si chip, but that won't cause shorts,
just no communications.

Are you getting any voltage on U7.4 across C77?

Also, just want to verify that you're probing the voltages on the pins themselves
as opposed to the solder pads.  It looks like everything is soldered pretty well
but I've seen failures with lifted pins before.


On 3/10/2019 1:54 PM, John AE5X wrote:
The area:


John AE5X
 

Yes John, I'm getting 1.4VDC across C77.

Don, ND6T
 

Next step would be to lift pin 5, slip insulation beneath, and test. Sounds like you have it cornered.

John AE5X
 

With pin 5 lifted I'm now showing the correct voltage. I was hoping I wouldn't as I would then know the problem :-(

So either something downstream is loading it down or is wasn't soldered properly in the first place. C73 and C76 are not shorted, neither are R56 or R57, and U28 pins 27 and 28 are not shorted to ground or each other.

I'll solder pin 5 back tomorrow, can't troubleshoot further tonight. Thanks for the suggestions, I appreciate it.

73 - John

Don, ND6T
 

It could still be the regulator. It looks like you decided not to socket U8? That socket would be very helpful for several reasons.
If you have a really good bench supply you could try substituting the 3.3 volts (with the regulator still isolated) by gently bringing it up from zero while monitoring the current. That would exonerate the regulator. I'm not sure what current would be normal but it is reasonable to assume that it will be below 30 milliamps. The rest of the board left de-powered, of course. If it acts like it will draw more than that, remove the display module and try again. My display came damaged internally so I am suspicious of it more than anything else.

John AE5X
 
Edited

I finally had time to return to this...

With the output pin of the 3.3V regulator lifted I get 3.7V as shown in photo above. With it re-soldered to the board, I only get 0.4V indicating to me that something downstream is loading it down or shorted to ground. Ohmmeter readings of the pad show that not to be the case - I get from 800k-ohms to ground on the output pad (this value rises as C73 and C76 charge as a result of the meter's voltage).

I'm baffled...

Don, ND6T
 

Two possibilities spring to mind: One is that the failure is a load with a breakdown voltage below 3.3 volts. That would look good on a low-voltage ohm meter but present an excessive current drain under operating voltage. This might be seen (or may not) by using the diode function on your DMM. Some DMMs have highter diode test voltages than others.
The other possibility is a bad regulator chip. It just might THINK that the load exceeds that 50ma rating and shuts down. Were you able to substitute the 3.3 volts and monitor the current as you increased it slowly from zero?
I would still suspect that the fault lies within the LCD display. I would remove it before anything else. Just a feeling.
Voltage regulators are extremely reliable.

Steven Weber
 

 

Unless the display is in the wrong way around, I doubt that’s it. It’s a shame you didn’t socket the processor as pulling that could be telling. I doubt it’s the regulator, those are nearly indestructible. Could be the Si5351 chip is in the wrong way around.

 

There are only three things connected to the 3.3V supply, the processor, the display and the Si5351. Sorry I didn’t make the 3.3V supply track more accessible otherwise you could cut the track between the different pieces and see which one is pulling down the supply. Does the regulator get hot?

 

 

 

Two possibilities spring to mind: One is that the failure is a load with a breakdown voltage below 3.3 volts. That would look good on a low-voltage ohm meter but present an excessive current drain under operating voltage. This might be seen (or may not) by using the diode function on your DMM. Some DMMs have highter diode test voltages than others.
The other possibility is a bad regulator chip. It just might THINK that the load exceeds that 50ma rating and shuts down. Were you able to substitute the 3.3 volts and monitor the current as you increased it slowly from zero?
I would still suspect that the fault lies within the LCD display. I would remove it before anything else. Just a feeling.
Voltage regulators are extremely reliable.

 

Don, ND6T
 

Lacking a good variable supply for testing, try this instead: With pin 3 of U7 still isolated, attach a 100 ohm resistor from it to ground while measuring the voltage on the pin. This will provide a 33 ma current load. The chip is rated to 50 ma. It should present a nice 3.3 volts. If not, then replace the bad chip. If the voltage holds at 3.3 volts, remove the display and test.

John AE5X
 
Edited

I will pull the processor out and re-install with the socket - didn't mean to skip the socket the first time around. I can remove it fairly easily with Chip-Quick that I have on hand. The Si5351 chip and display are installed correctly.